Shape memory alloys (SMAs) are metallic alloys that may recover apparent permanent strains when they are heated above a certain temperature. SMAs have two stable states or phases; a hot or austenite state and a cold or martensite state.
In the austenite state, the alloy is hard and rigid, while in the martensite state, the alloy is softer and flexible. In the martensite state, the SMA may be stretched or deformed by an external force. Upon heating, the SMA will return to its austenite state and contract or recover any reasonable stretch that was imposed on it. Thus, the SMA recovers with more force than was required to stretch it out. This exerted force upon contraction may be used to perform any number of tasks such as, but not limited to, turning a device on or off, opening or closing a device or object, or actuating a device or object.
A variety of problems, however, may occur when an SMA is incorporated in a device to perform one or more of these tasks. For example, SMAs can be damaged if they are inhibited from contracting when heated above their working range of austenite temperatures. In addition, abrupt loading of an SMA, such as short, high peaks of force, can also damage and reduce the longevity of an SMA. For example, an SMA actuator may be used to actuate a latch for a door or a lid. Typically, when the door or lid is closed, it pushes the latch out of the way. Once the door clears the latch, the latch will snap back into position and hold the door closed. Snapping back into position can jerk the SMA, causing a short high peak of force that damages the SMA. Additionally, an SMA that is used in an actuated devices that has two or more positions can droop and become caught or entangled in other parts of the device when the SMA is cooled and elongated but is not stretched taut or placed under tension.